ture of the extracted steam because a temperature difference of at least a
few degrees is required for any effective heat transfer to take place.
The condensed steam is then either pumped to the feedwater line or routed
to another heater or to the condenser through a device called a trap.A trap
allows the liquid to be throttled to a lower pressure region but trapsthe
vapor. The enthalpy of steam remains constant during this throttling process.
The open and closed feedwater heaters can be compared as follows. Open
feedwater heaters are simple and inexpensive and have good heat transfer
characteristics. They also bring the feedwater to the saturation state. For
each heater, however, a pump is required to handle the feedwater. The
closed feedwater heaters are more complex because of the internal tubing
network, and thus they are more expensive. Heat transfer in closed feedwa-
ter heaters is also less effective since the two streams are not allowed to be
in direct contact. However, closed feedwater heaters do not require a sepa-
rate pump for each heater since the extracted steam and the feedwater can
be at different pressures. Most steam power plants use a combination of
open and closed feedwater heaters, as shown in Fig. 10–17.
EXAMPLE 10–5 The Ideal Regenerative Rankine CycleConsider a steam power plant operating on the ideal regenerative Rankine
cycle with one open feedwater heater. Steam enters the turbine at 15 MPa
and 600°C and is condensed in the condenser at a pressure of 10 kPa.Chapter 10 | 571PumpBoiler
CondenserPumpTrap Trap TrapClosed
FWHClosed
FWHClosed
FWHOpen
FWHTurbineDeaeratingFIGURE 10–17
A steam power plant with one open and three closed feedwater heaters.